1. Field of the Invention
The present invention relates to a light-alloy casting heat treatment method and, more particularly, to a method of heat-treating a cylinder head consisting of an aluminum alloy.
2. Description of the Related Art
In the heat treatment of a light-alloy casting, a T6 process complying with JIS (Japanese Industrial Standard) is generally employed to largely increase the material strength. In the T6 process, a light-alloy casting (to be referred to as a work hereinafter) is heated at about 500.degree. C. and held at this temperature for several hours. After this, the work is hardened in water at room temperature or warm water and held at about 180.degree. C. for several hours.
However, since residual stress is generated in the work in hardening by the T6 process, the work cracks due to fatigue under a severe use environment.
The residual stress is traced back to temperature differences among different portions of the work. For example, during hardening, the cooling rate is high outside the work while it is low inside the work, and a temperature difference is generated between outside and inside the work. When the thermal stress due to this temperature difference exceeds the material proof strength of the work, residual stress is generated. Especially, because castings have complex shapes, the temperature readily varies locally, and the residual stress becomes high.
To reduce the residual stress, various methods have been proposed.
A polymer solution is used as a hardening refrigerant (Japanese Patent Application No. 2-62247).
The work shape or refrigerant circulation is improved to promote refrigerant supply to the work (Japanese Patent Laid-Open No. 4-136141).
The refrigerant temperature in hardening is increased (Sumitomo Light Metal Industries, Ltd., Technical Report Vol. 31, No. 2, 1990 (pp. 28-44).
The temperature in tempering is increased (Aluminum, Vol. 3, ASM (1967), 355).
Vibration is applied to the work after heat treatment (Papers of Japan Society of Mechanical Engineers, Vol. 52, No. 477, 1986, May).
However, these prior arts cannot sufficiently reduce the residual stress. Even if the residual stress can be reduced, the material strength lowers.